1. Field of the Invention
The present invention relates generally to power tools and electrical motor controllers for such tools. More particularly the invention relates to a microprocessor-based or microcomputer-based control circuit for monitoring and controlling various operating parameters of the tool.
2. Description of the Prior Art
In controlling the speed of an electric motor for use in power tools, it is now generally known to use gate electronic power controlling devices, such as a SCR's or triacs, for periodically transferring electrical energy to the motor. Many popular power tools employ universal motors which are readily controllable using such gate controlling devices.
Generally speaking, gated speed control circuits work by switching the motor current on and off at periodic intervals in relation to the zero crossing of the a.c. current or voltage waveform. These periodic intervals are caused to occur in synchronism with the a.c. waveform and are measured in terms of a conduction angle, measured as a number of degrees. The conduction angle determines the point within the a.c. waveform at which electrical energy is delivered to the motor. For example, a conduction angle of 180 degrees per half cycle corresponds to a condition of full conduction, in which the entire, uninterrupted alternating current is applied to the motor. Similarly, a 90-degree conduction angle corresponds to developing the supply voltage across the motor commencing in the middle of a given half cycle and thus corresponds to the delivery of approximately half of the available energy to the motor. Conduction angles below 90 degrees correspond to the transfer of even lesser quantities of energy to the motor.
Motor speed control circuits of the prior art have employed gating devices to alter the conduction angle in order to deliver a predetermined amount of energy to the motor, and to thereby achieve a predetermined motor speed. With universal motors, which are commonly used in power tools, motor speed is also related to the load placed on the motor. That is, under no load the motor delivers one given speed (the no load speed) and under load, the motor speed decreases as the load increases. The inverse relationship between speed (R.P.M.) and load (torque) at various conduction angles for a given motor may be expressed graphically as a family of curves in a speed-torque diagram.
One scheme for controlling motor speed simply selects a desired no load speed by selecting the appropriate conduction angle. The speed control circuit is of an open loop configuration, which means that no speed sensing mechanism is used to provide a feedback signal for maintaining the desired speed as the load is varied. Thus the open loop motor speed control circuit is capable of providing a preselected no load speed, but has no mechanism for holding speed constant under a changing load. In open loop, the motor speed will diminish in accordance with the speed-torque relationship as a load is applied to the tool. In the hands of a skilled operator, the open loop configuration provides a tool in which the power demands, and potentially destructive over heating conditions, can be sensed by the decrease in motor speed. However, such configurations do not provide for constant speed operation.
In contrast to the open loop configuration, some motor speed control circuits are designed as a closed loop configuration. In a closed loop configuration means are provided for sensing either the rotational speed of the motor or the current drawn by the motor to provide a feedback signal indicative of actual motor speed. The feedback signal is compared with an operator selected desired speed to determine an error signal. The error signal is then used to speed up or slow down the motor so that a substantially constant rotational speed is achieved. While closed loop motor speed control configurations offer the ability to operate a motor at a relatively constant speed, to a large extent independent of the load placed on the motor, they are not without problems.
One significant problem with closed loop motor speed control is the potential for overheating the motor under heavy loads at low speeds. Present day power tools use cooling fans, driven by the motor armature for dissipating heat generated by the motor. Such cooling fans become gradually less efficient as motor speed diminishes, to the point where overheating can become a significant problem. In a closed loop configuration, a power tool can be quite readily overheated when a desired speed corresponding to an armature speed insufficient to develop efficient fan cooling (e.g. below 10,000 RPM) is selected. Specifically, if the power tool is placed under a heavy load, the motor speed control circuit will increase the conduction angle, as the load on the motor is increased, in an effort to maintain a constant speed. This causes increasingly higher currents to flow through the windings of the motor with a dramatic rise in temperature. Without adequate fan cooling the tool quickly overheats which may cause permanent damage to the tool's lubricant-impregnated bearings or other components. Even in the hands of a skilled operator, it may not be readily apparent that an overheating condition is taking place until it is too late. The constant low operating speed can give a false impression that little power is being delivered to the motor, even when the power is in fact quite high due to the operation of the closed loop speed control circuit. In this state, over heating and damage can occur quite rapidly. Thermal protection circuits and over current protection circuits are known for combating the overheating problem, however, in order to fully protect against overheating, the sensitivity of these circuits must be high and thus quite often will falsely trigger a motor shut down when the operator is only momentarily overloading the tool, without any danger of permanent damage to the tool.
Another feature which is present in more sophisticated motor speed control circuits is an anti-kickback feature for removing power from the tool when an imminent kickback situation is detected. Generally, the kickback condition corresponds to a very rapid change in load, such as might occur when the tool grabs or seizes in a work piece, causing a backward thrust of the work piece or tool. Kickback problems are most significant with power tools which develop high torque. Several anti-kickback detection schemes have been proposed. One such anti-kickback scheme involves monitoring the rate of change in motor current, while another scheme involves monitoring the rate of change of motor speed. An example of a system which employs a rate of change of motor current detection scheme may be found in U.S. Pat. No. 4,249,117, to Leukhardt, issued Feb. 3, 1981. An example of a rate of change of motor speed detection scheme may be found in U.S. Pat. No. 4,267,914, to Saar, issued May 19, 1981. Both of the above noted patents are assigned to the assignee of the present invention.
While both kickback detection schemes have proven useful, it has heretofore been difficult to adapt such schemes to a wide range of operating speeds. In order to have sufficient sensitivity at higher operating speeds, the kickback sensing circuitry of the prior art may produce false kickback detections at lower operating speeds. Moreover, it has not heretofore been possible to readily adapt one kickback detecting scheme to a wide variety of power tools. In this regard, heavy duty half-inch drills, for example, have a high gear ratio and generate a lot of torque. For such drills a high kickback sensitivity is desirable. However, for quarter-inch drills, have a relatively low gear ratio and do not generate a lot of torque, rapid speed variations with change in loads are common and therefore the kickback sensitivity should be low. Prior art kickback detection schemes are not readily adaptable to different sensitivity settings for use with such broad ranges of tools.